Thought you should be aware of the following. Though I have owned the Sigma 120-400mm f/4.5-5.6 OS HSM lens for my Canon 7D for a couple of months now, and have been satisfied with the results, somehow I got the impression that the pictures were kind of soft, definitively software than those taken with Canon lenses. I attributed that to the fact that this lens is not Canon and as such, its optical performance was lower than that of Canon lenses.
However, browsing for pictures on the Internet taken with this same lens, I saw pin sharp pictures that I could not get. So, to make a long story short, after reading a lot on Internet, I discovered that this brand-new lens has a front-focus problem.
I corrected this problem on the 7D by applying +15 point of compensation on the camera settings. Now the lens delivers high-quality, pin-sharp pictures, and I am pleased
So if you own this Sigma lens for a Canon body, check it for front-focusing problems.